Contact alloys for vacuum circuit interrupters



United States Patent Int. Cl. C22c 9/00 US. Cl. 75153 1 Claim ABSTRACTOF THE DISCLOSURE In order to decrease the chopping current uponinterrupting the vacuum type of circuit interrupters as Wel as improvingthe property of resisting to welding of contacts, the disclosed contactalloys comprise from 10 to 20% by weight of cobalt, from 0.1 to 1.0% byweight of bismuth and the balance copper except for small amounts ofincident impurities. Contact alloy suitable for use as materials forcontacts of vacuum circuit interrupters adapted to interrupt relativelylow currents have also been disclosed comprising from to less than byweight of one element selected from the group consisting of bismuth,tellurium and lead, and the balance copper except for small amounts ofincident impurities. These contact alloys are relatively low in choppingcurrent characteristics and good in property of resisting to welding.

This invention relates to contact alloys for use as materials forelectric contacts of vacuum circuit interrupters operative to interrupta flow of current through the associated circuit through opening of thecontacts in a high vacuum established within the interrupter.

It is Well known that if electric contacts for use in vacuum circuitinterrupters are composed of copper that such copper contacts have anability to interrupt a high magnitude of alternating current. However itis also known that they are disadvantageous in high magnitudes ofchopping current resulting in an abnormal voltage induced in theassociated circuit upon interruption. There has been previouslyprevailing the concept that the chopping phenomenon is associated with ametallic vapor or vapors evolved upon arcing across electric contactsand that any metal high in vapor pressure or low in thermal conductivityhas a low magnitude of chopping current. Therefore, upon the basis ofthat concept there has been proposed the addition of a large amount of asuitable metal high in vapor pressure to another metal such as copperhigh in thermal conductivity in order to decrease a magnitude ofchopping current for the resulting contact alloy. For example, an alloyincluding 20% by weight of bismuth and the balance copper could beutilized as electric contacts for use in vacuum circuit interrupters.

However it is difiicult to seal alloys of the type containing a largeamount of metal high in vapor pressure into the associated vacuumcontainer. Also since low boiling point elements are generally lowmelting point elements, brazing of those alloys causes such a lowmelting point element to be diffused into the associated brazingmaterial during the brazing operation leading to the difficulty withwhich a sound brazed part is formed.

Therefore if any alloy containing a large amount of a low melting pointelement is used as a material for electric contacts which are, in turn,heated to an elevated temperature and subject to evacuation treament tobe held in a high vacuum space and which have applied thereto repeatedimpact forces during service, then troubles that might be caused fromthe use of such an alloy is particularly significant.

Accordingly, it is an object of the invention to provide a new andimproved copper base alloy for use as 3,502,465 Patented Mar. 24, 1970"ice a material for electric contacts of vacuum circuit interruptershaving a low magnitude of chopping current without the necessity ofadding a large amount of a low melting-point element thereto as well ashaving' the improved property of resisting to welding.

Briefly, the invention accomplishes the above object by the provision ofa copper base alloy comprising from 10 to 20% by weight of cobalt, from0.1 to 1.0% bY weight of bismuth and the balance copper except for smallamounts of incident impurities.

It is another object of the invention to provide a new and improvedcopper base alloy for use as a material for electric contacts of vacuumcircuit interrupters adapted to interrupt relatively low current, whichalloy is good in property of resisting to welding and also has therelatively low chopping current characteristics.

To this end, there is provided a copper base contact alloy comprisingfrom 5 to less than 10% by weight of one element selected from the groupconsisting of bismuth, tellurium and lead, and the balance copper exceptfor small amounts of incident impurities.

These and other objects and advantages of the invention will becomeapparent from the following detailed description.

The addition of a low melting point element such as bismuth, antimony orthe like, in an amount of 10% or more based upon the weight of theresulting alloy to copper is known effective for decreasing a choppingcurrent for such an alloy.

Extensive tests for the so-called chopping phenomenon were conductedwith copper base alloys containing one element selected from the groupconsisting of bismuth, tellurium and lead, in a varying amount. Electriccontacts made of such alloys were operated to interrupt alternatingcurrents having an eifective magnitude of 10 amperes at 80 volts and aneffective magnitude of 20 amperes at 160 volts in a vacuum of from 1 10-to 5X1O- torr. For each pair of electric contacts, a chopping magnitudeof arc current occurring upon interruption was measured ten times andboth the average of ten measured magnitudes and a maximum magnitude aretabulated in the following Table I. In Table I, for purpose ofcomparison data for copper are included.

TABLE I.--EFFECT OF ADDITION OF LOW MELTING ELE- MENTS UPON CHOPPINGCURRENT FOR COPPER Chopping current in A upon interrupting alternatingcurrent of specified efieetive magnitude Alloy in Average MaximumAverage Maximum percent wt. in in in A in A From the above Table I itwill be seen that the addition of one of bismuth, tellurium and lead inan amount of 1% or less based upon the weight of the alloy did notmaterially affect the magnitude of the chopping current for copper.

On the other hand, in order to determine the effect of TABLE IL-EFFECTOF ADDITION OF LOW MELTING ELEMENTS UPON WELDING POWER OF COPPERSeparating force in kg. after passage of current having specified peakmagnitude in ka.

Alloy in percent wt. 10 12 14 15 As seen in Table II, all the lowmelting elements do not always decrease the welding power. For example,copper-antimony alloys are inferior in property of resisting to weldingto pure copper. However Table II indicates that the addition of bismuth,tellurium or lead to copper improved the property of resisting towelding of the resulting alloys. It also indicates that an increase inamount of bismuth, tellurium or lead added to copper tends somewhat toincrease the welding power of the resulting copper base alloy. Thus ithas been found that bismuth, tellurium or lead is preferably added in anamount of at most 1% by weight to copper.

According to the invention, one element selected from the groupconsisting of bismuth, tellurium and lead is added in a small amount tocopper only for the purpose of improving the property of resisting towelding and a decrease in chopping current relies upon the use ofanother element. The chopping current tests were conducted with numerousbinary alloys containing copper and iron, cobalt or nickel in a varyingamount respectively in the same manner as previously described inconjunction with Table I. Their results are tubulated in the followingTable III. For purpose of comparison Table III include further data foriron, cobalt and nickel.

TABLE III.-CHOPPING CURRENT FOR COPPER BASED ALLOYS CONTAINING ONE OFIRON, COBALT AND NICKEL Chopping current in A upon interruptingalternating current of specified effective magnitude Alloy in I percentwt. Average Maximum Average Maximum From the above Table III it will beseen that from 10 to 20% by weight of cobalt is effectively added tocopper for purpose of decreasing the chopping current for the resultingalloy and that the addition of iron or nickel similar in properties tocobalt is not so effective as the addition of cobalt. As will be wellknown, nickel is higher in vapor pressure than cobalt and copper-nickelalloys are less in thermal conductivity than copper-cobalt alloys. Alsoa binary copper-nickel alloy including approximately 50% of nickel basedupon its weight has a minimum thermal conductivity. Nevertheless thechopping current decreases monotonously as the content of nickelincreases. Thus it has been found to be incorrected that the magnitudesof chopping current of such alloys are estimated simply and easily onthe basis of both the properties of each of the elements composing thealloys and the general physical characteristics of the alloys.

While cobalt is effective for decreasing the chopping current as abovedescribed it has been found that the same is disadvantageous in anincrease in welding power. However this disadvantage has been eliminatedthrough the addition of bismuth in a small amount to the coppercobaltalloys as seen in the following Table IV. The figures listed in Table IVwere obtained in the same manner as already described in conjunctionwith Table II. Table IV also includes data for a Cu-10% Co alloy and aCu-20% Co alloy for purpose of comparison.

TABLE IV.WELDING POWER OF COPPER-COBALT ALLOY CONTAINING ONE OFBISMU'IH, TELLURIUM AND LEAD Welding power in kg. after passage ofcurrent having specified peak magnitude in ka.

Alloy in percent wt. 10 12 14 15 Cu, 10 O0 100 100 100 100 Cu, 10 Co,0.01 Bi 19 77 67 100 Cu, 10 G0, 0.1 B 9 18 28 100 Cu, 10 CO, 0.5 B 14 1812 22 Cu, 10 CO, 1 Bi 22 18 26 30 Cu, 10 Co, 3 Bi 9 15 38 48 Cu, 10 C0,5 Bi 36 38 33 54 Cu, 10 Co, 0.01 Pb 100 100 100 100 Cu, 10 Co. 0.1 Pb...66 29 69 100 Cu, 10 Co, 0.5 Pb, 24 56 86 81 Cu, 10 C 1 Pb 2 63 100 100Cu, 10 C 3 Pb 19 29 35 69 Cu, 10 C 1 Te 36 36 86 100 011, 10 O0, 3 Te 3659 100 100 Cu, 20 C0 100 100 100 100 Cu, 20 G0. 0. 24 43 100 100 Cu, 20,0- Bi 8 4G 21 17 Cu, 20 C 0.5 B1. 10 12 35 24 Cu, 20 C 1 Bi--. 18 32 1620 Cu, 20 0 .3 Bi 8 59 23 28 Cu, 20 oo, 1 Te- 24 1s 97 100 Cu, 20 O0, 3Te 29 61 62 100 From Tables II and IV it is noted that tellurium asadded to copper to form binary alloys effectively decreases the weldingpower thereof whereas tellurium as added to copper-cobalt alloys is lessefiective. From Table IV it will be also seen that, in order to decreasethe welding power of the copper base alloy lead is required to be addedto the alloy in a large amount as compared with bismuth. However theelfect of lead is not so definite as the effect of bismuth added to thealloy. It has been found that a content of bismuth required forimproving the property of resisting to welding of the contact alloysshould range from 0.1 to 1.0% based upon the weight of the alloy. Thecontent of bismuth in excess of the magnitude just specified does notimprove such property and instead only adversely aifects the workabilityof the resulting alloys.

The invention also contemplates to provide binary copper base alloyscontaining bismuth, tellurium or lead respectively and especiallysuitable for use as materials for electric contacts of vacuum circuitinterrupters adapted to interrupt relatively low currents which contactsare relatively good in property of resisting to welding and relativelylow in chopping current characteristics.

As understood from Table I, the copper-bishmuth, copper-tellurium andcopper-lead alloys each are unsuitable for decreasing the choppingcurrent, provided that the alloys contain copper in an amount exceedingbased upon the weight of the alloy. On the other hand, it will beapparent from Table II that in order to improve the property ofresisting to welding the content of bismuth should be less than 10% byweight for copperbismuth alloys whereas the content of tellurium mayrange from 1 to 10% by weight and if desired, may exceed 10% forcopper-tellurium alloys. Also, for copperlead alloys, the content oflead may range from 1 to 10% by weight or more.

It has been found that for the binary copper-bishmuth alloys, theoptimum composition should contain from 5 to less than by weight ofbismuth and the balance copper. Only considering the chopping currentcharacteristics and the property of resisting to welding coppertelluriumand copper-lead alloys may be sufficiently uti lized including at most90% by weight of copper. As previously described, however, the use of ahigh vapor pressure element such as tellurium, lead or the like in alarge amount leads to various harmful effects. Thereforecopper-tellurium and copper-lead alloys should also include from 5 toless than 10% by weight of tellurium and leads respectively and thebalance copper.

It is to be understood that the present alloys are allowed to includesmall amounts of incident impurities provided that they do not exhibitthe adverse effects upon the electric contacts of vacuum circuitinterrupters as gaseous elements.

From the foregoing it will be appreciated that the invention providescontact alloys comprising from 10 to by weight of cobalt, from 0.1 to10% by weight of bismuth and the balance copper except for small amountsof incident impurities, which alloys are low in chopping current,excellent in property of resisting to welding enough to be used asmaterials for heavy current contacts and also suitable for use asmaterials for contacts of vacuum circuit interrupters adapted to be usedin producing envelopes of electron tubes.

The ternary copper-cobalt-bismuth alloys were tested in the same manneras previously described in conjunction with Table I for chopping currentand the results are given in the following Table V.

TABLE V.CHOPPING CURRENT OF TERNARY ALLOY OF INVENTION Chopping currentin A upon interrupting alternating current of specified electricmagnitude 10A 20A Alloy in percent wt. Average Maximum Average MaximumThe alloys of the invention are produced by vacuum melting followed byvacuum cooling.

Copper-cobalt alloys are generally liable to segregate duringsolidification and a content of cobalt in excess of 20% is diflicult toprovide a homogeneous alloy. It has, however, been found that thissegregation can be advantageously utilized to braze the electriccontacts of the present ternary alloys to the associated members. Morespecifically, upon producing the ternary alloys comprising from morethan 20% to 50% by weight of cobalt and bismuth in an amount as abovespecified, the resulting ingot will have an initially solidified surfacelayer enriched in cobalt but short of copper and containing no bismuth.This surface layer can then be utilized to effect brazing of contacts ineasy manner. If desired, a part of cobalt may be replaced by iron.

Also since the solubility of carbon to the alloy increases with thecontent of cobalt, carbon may be used to remove oxygen from the alloyduring the vacuum melting operation whereby an alloy very low incontents of gases can be more easily provided.

What I claim is:

1. An electrical contact for use in a vacuum circuit interrupterconsisting of from 10 to 20% by weight of cobalt, from 0.1 to 1.0% byweight of bismuth, and the balance copper except for small amounts ofincident impurities.

References Cited UNITED STATES PATENTS 2,975,255 3/1961 Lafferty 153 X2,975,256 3/1961 Lee et al. 200166 X 3,246,979 4/1966 Lafierty et al75153 X FOREIGN PATENTS 6,614 3/1965 Japan.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 200144, 166

